SURFACE-WAVE PROPAGATION ACROSS THE MEXICAN VOLCANIC BELT AND THE ORIGIN OF THE LONG-PERIOD SEISMIC-WAVE AMPLIFICATION IN THE VALLEY OF MEXICO

A network of nine broad-band seismographs was operated from March to M ay 1994 to study the propagation of seismic waves across the Mexican V olcanic Belt (MVB) in the region of the Valley of Mexico. Analysis of the data from the network reveals an amplification of seismic waves in a wide period band at the stations situated in the southern part of t he MVB. The group velocities of the fundamental mode of the Rayleigh w ave in the period range 2-13 s are found to be lower in the southern p art of the MVB than in its northern part and in the region south of th e MVB. The inversion of dispersion curves shows that the difference in group velocities is due to the presence of a superficial low-velocity layer (with an average S-wave Velocity of 1.7 km s(-1) and an average thickness of 2 km) beneath the southern part of the MVB. This low-vel ocity zone is associated with the region of active volcanism. Numerica l simulations show that this superficial low-velocity layer causes a r egional amplification of 8-10 s period signals, which is of the same o rder as the amplification measured from the data. This layer also incr eases the signal duration significantly because of the dispersion of t he surface waves. These results confirm the hypothesis of Singh et al. (1995), who suggested that the regional amplification observed in the Valley of Mexico is due to the anomalously low shear-wave velocity of the shallow volcanic rocks in the southern MVB